A central problem in metazoan development is how cells are governed to grow or differentiate in a tissue-specific pattern. We propose to analyze the genetic and molecular basis of this fundamental problem in the germ line of the nematode Caenorhabditis elegans. The C. elegans germ line is particularly suitable for these studies because of its powerful genetics, completed genome sequence, and cellular simplicity. A single somatic regulatory cell serves as a "niche" for germline stem cells, and this cell promotes germline proliferation by the conserved GLP-1/Notch signaling pathway. Moreover, the mitosis/meiosis decision is controlled, at least in part, by RNA-binding proteins of four conserved classes (Puf, Nanos, STAR/quaking, Bic-C) plus a newly discovered cytoplasmic poly(A) polymerase. In the next five years, we propose to analyze mechanisms by which central players of germline control (GLP-1 signaling/RNA regulators) are able to orchestrate growth of the germ line and specification of the mitosis/meiosis decision. First, we will identify genes directly controlled by GLP-1 signaling to promote germline mitoses. This work is predicted to link GLP-1 signaling with the RNA regulatory network and perhaps with the cell cycle machinery driving mitosis. Second, RNA regulators will be analyzed with respect to their relationship to GLP-1 signaling, their relationships among each other and their target mRNAs. This work is predicted to delineate the backbone of the genetic circuit controlling germline growth and the switch into the meiotic cell cycle. Third, additional regulators will be identified and investigated to learn their genetic and molecular relationships with the known central players. To this end, we propose to combine bioinformatics, RNAi and traditional genetics. Fourth, the nature of germline stem cells within the germline mitotic region will be investigated. Finally, the molecular circuit controlling germline growth and differentiation will be linked to cellular behaviors in the mitotic germ line, including cell cycle progression and tissue shape. The proposed studies will serve as a paradigm for developmental controls in higher organisms, including humans. The health-relatedness of this work is two-fold. First, the germ line holds the key to fertility. Second, germline stem cells may be the ultimate source of stem cells for tissue replacement in diseased or injured individuals.